Multilayer composite coatings, for example, color-plus-clear coating systems, involving the application of a colored or pigmented basecoat to a substrate followed by application of a transparent or clearcoat over the basecoat, have become increasingly popular as original finishes for a number of consumer products including, for example, automotive vehicles. The color-plus-clear coating systems have outstanding appearance properties such as gloss and distinctness of image, and provide excellent coating systems such as corrosion resistance, scratch and abrasion resistance, and resistance to deleterious environmental conditions such as acid rain. Such color-plus-clear coating systems have become popular for use on automotive vehicles, aerospace substrates, floor coverings such as ceramic tiles and wood flooring, packaging materials and the like.
A conventional automotive coating process typically includes the sequential application of an electrodepositable coating composition, usually a cationic composition, a primer-surfacer coating composition over the electrodeposition coating, a color-enhancing and/or effect-enhancing basecoating composition over the primer-surfacer coating, and a transparent or clear coating composition over the basecoat. In some instances, the electrodeposition coating is applied over a mill-applied weldable, thermosetting coating which has been applied to the coiled steel metal substrate from which the autobody (or autobody parts, such as fenders, doors, and hoods) are formed.
For example, as mentioned above, on most automotive coating lines, the auto body is first given an electrodeposition coating commonly formed from a cationic electrodepositable coating composition. This electrodeposition coating typically is then thermally cured. The electrodeposition coating must be fully adherent to the substrate and inhibits corrosion of the substrate to which it is applied. In conventional electrodeposition coatings, the excellent adhesion and corrosion resistance properties can be derived from the inclusion in the electrodepositable composition of ionic film-forming resins and/or crosslinking agents which can comprise aromatic moieties. While providing excellent adhesion and corrosion resistance, these resins and/or crosslinking agents can be susceptible to degradation by visible and/or ultraviolet light which can penetrate through the subsequently applied coating layers. Such photodegradation can result in delamination of the electrodeposition coating from the substrate, causing catastrophic failure of the multilayer composite coating system.
A primer-surfacer coating composition typically is applied to the cured electrodeposition coating, and the primer-surfacer coating is then thermally cured. The primer-surfacer coating composition usually comprises a polymer composition which provides a tough and flexible coating; and typically is heavily pigmented, for example, with filler pigments, such as talc and clay, and often contains photodegradation-resistant pigments, for example, carbon black. The cured primer-surfacer coating layer can have a film thickness as high as 100 micrometers, but usually between 25 and 50 micrometers. As such, the primer-surfacer coating can enhance chip resistance of the multilayer composite coating system, and also can mask any surface defects present in the electrodeposition coating, thereby ensuring a smooth appearance of the subsequently applied top coatings. Moreover, the primer-surfacer affords visible and ultraviolet light opacity to prevent photodegradation of the previously applied electrodeposition coating. One known primer-surfacer is GPX 45379 commercially available from PPG Industries, Inc. of Pittsburgh, Pa.
A basecoating composition, most often an aqueous composition, then is applied to the cured primer-surfacer coating. The basecoating composition usually contains color-enhancing and/or effect-enhancing pigments.
The basecoating is typically given a flash bake at a temperature and for a time sufficient to drive off excess solvents, but insufficient to cure the basecoating composition. A transparent or clear coating then is applied to the uncured basecoating. This is commonly referred to as a wet-on-wet application. The clear coat can provide excellent gloss and distinctive of image, as well as scratch and mar resistance, and resistance to harsh environmental conditions.
In one known coating line, the substrate is electrocoated at an electrocoating station and then is moved into a primer zone for application of the primer-surfacer. As described above, the primer-surfacer is typically a relatively thick coating to mask surface defects in the underlying substrate. The applied primer-surfacer layer is cured and then the cured primer-surfacer can be sanded to remove surface defects and to provide a smooth outer surface for the application of further coatings. However, this sanding process can result in small particles of grit or dirt that must then be brushed or tacked off of the substrate before further coatings can be applied. After this tacking process, the substrate is moved into a basecoating zone where the fully color-pigmented basecoat composition is applied onto the cut-in portions of the substrate. The same fully color-pigmented basecoat composition is applied onto the primer-surfacer over the exterior of the substrate at one or more subsequent basecoat stations. The applied basecoat compositions are then baked to pre-dry the basecoating, and a clearcoat composition is applied onto the basecoat on the substrate exterior. Typically, the clearcoat composition is not applied onto the basecoat in some areas in the cut-in portions.
Due to the resultant cost-savings, there has been recent interest in the automotive coatings market in reducing the cured film thickness of one or more of the coating layers in the multilayer composition coating, and/or eliminating one or more of the coating steps altogether. For example, in some multilayer coating processes the primer-surfacer coating application and curing steps can be eliminated. That is, the basecoating composition is applied directly onto the cured electrodeposition coating. In such modified coating processes, both the electrodeposition coating and the basecoating are required to meet stringent durability, appearance, and physical properties specifications.
Further, as previously mentioned, for some applications, a weldable, corrosion inhibitive primer coating is mill-applied to metallic substrates. The basecoating composition can then be applied directly to the cured weldable primer coating with no intervening electrodeposition coating and no primer-surfacer coating.
Also, automotive parts and accessories, for example non-metal or elastomeric autobody parts, such as bumpers and body side moldings, typically are coated “off site” and shipped to the automobile assemble plants. Such substrates do not require corrosion resistance as do the metallic substrates discussed above. Hence, the basecoating composition can be applied directly to the non-metal substrate surface, or, alternatively, to a previously applied intervening adhesion-promoting primer coating.
U.S. Pat. No. 6,221,949 B1 discloses a coating formulation for use in multicoat paint systems which comprises a water-dilutable polyurethane resin having an acid number of 10 to 60 and a number average molecular weight of 4000 to 25,000. The polyurethane is prepared by reacting a polyester and/or polyether polyol having a number average molecular weight of 400 to 5000 or a mixture of such polyesters and polyether polyols; a polyisocyanate or mixture thereof; a compound which has in the molecule at least one group reactive toward isocyanate groups and at least one group capable of forming anions or a mixture of such compounds; and optionally a hydroxyl and/or amino-containing organic compound having a molecular weight of from 40 to 400, and at least partially neutralizing the resulting reaction product. The composition further comprises pigments and/or fillers where the ratio of binder to pigment is between 0.5:1 to 1.5:1. In such compositions, the presence of talc is required in an amount of 20 to 80% by weight of the overall quantity of pigment. This composition is employed in a process for forming a multicoat paint system in which the substrate is coated with an electrodeposition coating which is optionally predried or baked, the composition described above is applied to the electrodeposition coating and optionally predried without baking, a second aqueous coating is applied to the coating formed from the previously described composition and optionally predried without baking, a transparent coating is applied to the coating formed from the second aqueous composition, and the overall paint system is baked.
U.S. Pat. No. 5,976,343 discloses a process for multicoat lacquering of a substrate with a stoved first electrodeposition layer by a applying a second surface coating layer having a dry thickness of 10 to 30 microns consisting of a base lacquering agent containing a first water-based polyurethane resin, and wet-on-wet application of a third coating agent with a dry layer thickness of 7 to 15 microns. The third coating layer consists of a second water-based lacquering agent containing a polyurethane resin. A clear lacquering layer is then applied without stoving of the third coating agent, and the multicoat system is stoved to mutually cure the second, third and clear lacquer layers. The first base lacquering agent has a higher concentration of polyurethane resin than does the second base lacquering agent. Further, the patent discloses that the first base lacquering agent is prepared from the second base lacquering agent by admixing an appropriate amount of polyurethane resin with the second base lacquering agent.
U.S. Pat. No. 4,820,555 discloses a method for forming a multicoat system on a substrate by first applying an electrocoating composition to a substrate and curing the electrocoating composition, applying a sealercoating composition over the electrocoat and, optionally, baking the sealercoating, applying a metallic basecoating composition over the sealercoating, either drying, flash-baking, or curing the metallic basecoating, applying a clearcoating composition over the metallic basecoating, and baking the multicoat system. The sealercoating composition can be solvent-based or water-based, and provides improved metallic pigment orientation, basecoat smoothness and adhesion.
In an attempt to alleviate some of the problems associated with known coating processes, another coating line has been developed in which primer-surfacer application has been eliminated. However, in this process the structure and operation of the coating line must be significantly altered in order to accommodate problems arising from this change. For example, in this process after application of the electrodeposition coating, a first basecoat composition is applied over the exterior surface of the substrate. This first basecoat composition is a chip resistant, color pigmented composition that can be color keyed to approximate the desired final color of the coated substrate. The first basecoat composition is then heated to pre-dry the first basecoat and a second basecoat composition of the desired final color pigmentation is applied onto the first basecoat composition on the exterior surface. The cut-in portions are coated with the second basecoat composition, between application of the first and second basecoating composition. This modification is required due to the color transition areas that would be visible if the cut-in portions were coated first, as in a typical coating process. However, this change in the coating sequence means that this process is not easily incorporated in existing coating lines that are set up to coat the cut-in portions of the substrate before the exterior portions. Added expense must be incurred to either build a new coating line to practice this process or to modify an existing line to move the cut-in application to the end of the basecoating zone.
In view of the foregoing, it would be advantageous to provide a process for forming a multilayer composite coating system which eliminates the application and curing of a primer-surfacer coating whereby a first basecoating composition can be applied directly to an electrodeposition coating, or, alternatively, to a treated or untreated substrate; followed by wet-on-wet application of a second color-or effect-enhancing basecoat, the composition of which can be the same or different from that of the first basecoating composition, with subsequent wet-on-wet application of a clearcoat. Further, it is desirable that such a multilayer composite coating system be applied on a conventional coating line without significant modification.